The present invention relates to an infrared solid-state image sensor, and, more particularly, to an uncooled infrared solid-state image sensor and a manufacturing method thereof.
Infrared image sensing is characterized in its ability to pick up images at night as well as during the daytime and its higher transmittance to smoke and fog than visible radiation. Being capable of acquiring temperature information of an object to be sensed, infrared image sensors are adaptable to a variety of applications, such as a monitor camera and a fire detection camera, besides the field of defense.
Recently extensive studies have been made on "uncooled infrared solid-state imaging elements" which eliminate the need for the cooling mechanism to permit a low-temperature operation, which is the biggest shortcoming of a photoelectric conversion type infrared solid-state image sensor or a leading conventional type. The following infrared solid-state image sensors have been proposed.
(1) An infrared solid-state image sensor which uses, as a thermoelectric conversion element, a thermopile for converting a temperature difference to a potential difference through the Seebeck effect (Toshio Kanno, et al., Proc. SPIE vol. 2269, pp. 450-459, 1994).
(2) An infrared solid-state image sensor which uses, as a thermoelectric conversion element, a bolometer for converting a temperature change to a change in resistance based on the temperature change of a resistor (R. A. Wood, Proc. IEDM, pp. 175-177, 1993).
(3) An infrared solid-state image sensor which uses, as a thermoelectric conversion element, a pyroelectric element for converting a temperature change to charges based on the pyroelectric effect (Charles Hanson, et al., proc. SPIE Vol. 2020, pp. 330-339, 1993).
Of those three thermoelectric conversion systems, a thermopile element which uses a polysilicon film as a thermoelectric conversion material appears advantageous from the viewpoint of fabrication due to its feature that the entire manufacturing process for the element can be implemented by the existing silicon process.
For a thermopile type, the sensitivity R of an infrared solid-state image sensor is expressed by the following equation (see, for example, Paul w. Kruse, Proc. SPIE, Vol. 2552, pp. 556-563). EQU R=N.multidot.S.multidot..eta./{G(1+.omega..sup.2 .tau..sup.2).sup.1/2 }=N.multidot.S.multidot..eta./(G.sup.2 +.omega..sup.2 C.sup.2).sup.1/2(1)
where
N: the number of series connections of the thermopile, PA1 S: Seebeck coefficient of the thermopile, PA1 .eta.: infrared absorptance, PA1 G: thermal conductance, PA1 .omega.: modulation angular frequency of incident infrared rays, PA1 .tau.: thermal response time (.tau.=C/G), and PA1 C: heat capacity of the thermoelectric conversion section. PA1 Ib: bias current, PA1 .alpha.: temperature coefficient of a resistor (=(1/Re)(dRe/dT)), and PA1 Re: element resistance. PA1 G0(Si): thermal conductivity of polysilicon (=29 W/K/m), PA1 Spoly: cross-sectional area of polysilicon [m.sup.2 ], and PA1 Lpoly: length of polysilicon between a hot junction edge and a cold junction edge [m]. PA1 Wpoly: width of polysilicon [m], PA1 Hpoly: height of polysilicon [m], PA1 M: mass of the hot junction [kg], and PA1 g: gravity [m/sec.sup.2 ] PA1 E(Si): Young's modulus of silicon (=113.0 [GN/m.sup.2 ]}.
The sensitivity R for a bolometer type is given by: EQU R=Ib.multidot..alpha..multidot.Re.multidot..eta./{G(1+.omega..sup.2 .tau..sup.2).sup.1/2 } (2)
where
The sensitivity may be improved by increasing the bias current Ib. In this case, however, it is necessary to consider the influence of self heating originating from the energy consumption of Ib.sup.2 .multidot.Re.
The thermal terms in the equations 1 and 2, which both include the thermal conductance G and the heat capacity C, are expressed by the following equation. EQU Rt=1/{G(1+.omega..sup.2 .tau..sup.2).sup.1/2 }=1/(G.sup.2 +.omega..sup.2 C.sup.2).sup.1/2 (3)
It is apparent from this equation that reducing the thermal conductance G and the heat capacity C is important to improve Rt. In a low-frequency region, particularly, the sensitivity is improved by reducing the thermal conductance G. The heat capacity C, on the other hand, greatly contributes to the sensitivity in a high-frequency region or so-called response.
The thermal conductance G and heat capacity C in the equations 1 to 3 include not only values which are inevitable to transfer the output signal from the thermopile or bolometer to the semiconductor substrate, but also the values of the thermal conductance G and heat capacity C which are originated from the material layer that is present between the thermoelectric conversion material and the semiconductor substrate.
Because of those reasons, to thermally isolate the thermoelectric conversion element from the semiconductor substrate for temperature information readout and suppress an increase in thermal conductance G, uncooled infrared solid-state image sensors employ a cavity structure constructed by forming a cavity in the semiconductor substrate (see FIG. 1), or a cavity structure provided by forming a sacrificial layer in a multilayer structure on the semiconductor substrate and then etching that layer out (see FIG. 2), or connecting thermoelectric conversion elements, thermally isolated from one another, to the semiconductor substrate by bumps. Those thermal isolation structures achieve a high sensitivity.
A typical scheme of enhancing the output voltage of a thermopile is to connect N pairs of thermocouples in series as shown in FIG. 3 (N=12 in this example).
In principle, however, the thermopile infrared solid-state image sensor, which is considered advantageous from the manufacturing viewpoint due to the aforementioned feature of permitting the use of the silicon process, suffers a disadvantage on the thermal isolation structure for the following reason.
The bolometer type which detects a temperature change from a change in the resistance of a resistor should only have a pair of wires formed between the heat sensing side and the heat sink side as means for detecting a resistance change. The bolometer type therefore needs merely two heat transfer paths between the heat sensing portion and the heat sink. The thermopile element, by contrast, operates based on the Seebeck effect which converts a temperature difference between a hot junction and a cold junction of a thermocouple to a potential difference. To increase the potential difference thus requires series connection of thermocouples. As a result, the thermocouples themselves require heat transfer paths, which are twice number of the thermocouples to be connected in series, between the hot junction and cold junction.
In the case of the thermopile type, in particular, it is significantly important to suppress increases in the thermal conductance G and heat capacity C, which are caused by the material of the thermocouples as the thermocouples are series-connected.
The thermal conductance Gpoly produced by polysilicon or the material for the thermocouples is given by: EQU Gpoly=2N.multidot.G0(Si).multidot.Spoly/Lpoly (4)
where
As apparent from the above, a design technique of realizing a longer Lpoly for the same pixel size is considerably important.
The aforementioned scheme of connecting a plurality of thermocouples in series however suffers not only an increase in the number of heat transfer paths but also its inability to provide sufficiently long Lpoly. Therefore, the thermal conductance of a single heat transfer path cannot be sufficiently reduced.
In the thermoelectric conversion structure in above pixel, the hot junction of a thermocouple and the structure of thermally isolating an infrared ray absorbent, connected to the thermocouple, from the wiring portion of the thermocouple are not often considered conventionally. If the infrared ray absorbent and the wiring portion of the thermocouple both exist on the same diaphragm, for example, the thermal conductance between the hot junction of the thermocouple and the semiconductor substrate increases. To ensure a higher sensitivity, therefore, some kind of measure should be taken.
The use of a silicon oxide film or silicon nitride film for a support layer needed to construct the aforementioned cavity structure has already been reported (Toshio Kanno, et al., Proc. SPIE vol. 2269, pp. 450-459, 1994, and R. A. Wood, Proc. IEDM, pp. 175-177, 1993). As micronization of the thermoelectric conversion material progresses in accordance with reduction in pixel size, the influence of increases in the thermal conductance G and the heat capacity C, caused by the presence of the support layer, on the sensitivity cannot be neglected.
Even with the same cavity structure being formed, therefore, the thermopile element which has multiple heat transfer paths formed by the thermoelectric conversion elements themselves has a higher thermal conductance G between the thermoelectric conversion element and the semiconductor substrate, as compared with the bolometer element. This results in a lower sensitivity.
A noise equivalent temperature difference (NETD), one important characteristic besides the sensitivity, is proportional to the reciprocal of the product, Ad.multidot.R, of the detected area Ad and the sensitivity R. To reduce the pixel size of an infrared solid-state image sensor, therefore, it is understood that merely keeping the sensitivity R is not enough but further improvement of the sensitivity R is needed to compensate for a drop of NETD which may result from the reduction of the detection area Ad.
According to the conventional processing technique, Spoly is reduced by making the polysilicon film thinner. As the polysilicon width is limited by the photolithography technology, however, Spoly cannot be made sufficiently small.
For a fine pixel structure, to reduce the heat capacity C of the hot junction of thermocouples and reduce the thermal conductance G, it is effective for the thermocouples above the cavity structure to support by themselves.
The self-supporting of the thermocouples above the cavity structure requires that the thermocouples should have a mechanical strength as a support member.
In consideration of the polysilicon-based thermocouple structure used in the equation 4, a change Z in the gravitational direction at a hot junction when a load is applied to the hot-junction side of polysilicon secured to a cold junction edge is given by the following equation. For the sake of simplicity, the cross-sectional structure is rectangular. EQU Z=(4Lpoly.sup.3 /Wpoly.multidot.Hpoly.sup.3).times.{M.multidot.g/E(Si)}(5)
where
As apparent from the equation 5, for the same cross-sectional area, it is very important to make Wpoly smaller and Hpoly larger in order to prevent deformation of the thermocouples.
This tendency should become particularly important in further miniaturization of pixels that people are unquestionably trying very hard to achieve. According to the conventional technique, however, rather the opposite tendency takes place for Spoly, so that a support layer of oxidation silicon or silicon nitride should be formed at the price of the increased thermal conductance G and heat capacity C.
Because, as apparent from the above, a thermal conductance between the hot junction of a thermocouple and the semiconductor substrate of the conventional uncooled infrared solid-state image sensor cannot be made sufficiently small, an improvement on the sensitivity is limited. While there is an attempt to connect thermocouples in series to improve the sensitivity, the series connection of the thermocouples inevitably leads to increases in the thermal conductance and heat capacity caused by the thermocouples themselves. This attempt thus has difficulty in achieving a sufficient sensitivity.